Writing to an External Program

You can instruct tar to send the contents of each extracted
file to the standard input of an external program:

‘--to-command=command’

Extract files and pipe their contents to the standard input of
command. When this option is used, instead of creating the
files specified, tar invokes command and pipes the
contents of the files to its standard output. The command may
contain command line arguments (see Running External Commands,
for more detail).

Notice, that command is executed once for each regular file
extracted. Non-regular files (directories, etc.) are ignored when this
option is used.

The command can obtain the information about the file it processes
from the following environment variables:

TAR_FILETYPE

Type of the file. It is a single letter with the following meaning:

f

Regular file

d

Directory

l

Symbolic link

h

Hard link

b

Block device

c

Character device

Currently only regular files are supported.

TAR_MODE

File mode, an octal number.

TAR_FILENAME

The name of the file.

TAR_REALNAME

Name of the file as stored in the archive.

TAR_UNAME

Name of the file owner.

TAR_GNAME

Name of the file owner group.

TAR_ATIME

Time of last access. It is a decimal number, representing seconds
since the Epoch. If the archive provides times with nanosecond
precision, the nanoseconds are appended to the timestamp after a
decimal point.

TAR_MTIME

Time of last modification.

TAR_CTIME

Time of last status change.

TAR_SIZE

Size of the file.

TAR_UID

UID of the file owner.

TAR_GID

GID of the file owner.

Additionally, the following variables contain information about
tar mode and the archive being processed:

These variables are defined prior to executing the command, so you can
pass them as arguments, if you prefer. For example, if the command
proc takes the member name and size as its arguments, then you
could do:

$ tar -x -f archive.tar \
--to-command='proc $TAR_FILENAME $TAR_SIZE'

Notice single quotes to prevent variable names from being expanded by
the shell when invoking tar.

If command exits with a non-0 status, tar will print
an error message similar to the following:

tar: 2345: Child returned status 1

Here, ‘2345’ is the PID of the finished process.

If this behavior is not wanted, use ‘--ignore-command-error’:

‘--ignore-command-error’

Ignore exit codes of subprocesses. Notice that if the program
exits on signal or otherwise terminates abnormally, the error message
will be printed even if this option is used.

‘--no-ignore-command-error’

Cancel the effect of any previous ‘--ignore-command-error’
option. This option is useful if you have set
‘--ignore-command-error’ in TAR_OPTIONS
(see TAR_OPTIONS) and wish to temporarily cancel it.